1. Field of the Invention
This invention relates to the deposition of carbon-containing layers and, more particularly, to effusion cells and methods for their use in the molecular beam epitaxial (MBE) growth of such layers.
2. Discussion of the Related Art
Molecular beam deposition of layers of material (e.g., semiconductors, metals, insulators, or superconductors) on a heated substrate in an ultra high vacuum is well known in the art. In particular, MBE is one of the principal techniques used in the semiconductor device industry to fabricate high quality, single crystal, semiconductor layers with thickness control on the order of a monolayer. In MBE a single crystal substrate or wafer is placed in a vacuum chamber where it is heated. Effusion cells loaded with source materials in solid or liquid form are heated to vaporize the material and generate beams of constituent atoms, which are directed at the substrate. Alternatively, one or more of the effusion cells may be replaced by a gas jet coupled to a source of gaseous material to generate one or more of the requisite beams. (The latter deposition technique is known as chemical beam epitaxy, or CBE, especially if a chemical reaction occurs on the substrate surface during, or just before, incorporation of a component of the beam.) In both MBE and CBE the constituent atoms adsorb on the substrate surface and incorporate into the underlying crystal structure to form a layer. Control is so good that the layer is literally formed one monolayer at a time.
Although the term molecular is used to describe the vaporized source material in this deposition process, those skilled in the art understand that the source material may be elemental (or atomic) as well as compound (or molecular).
In the MBE growth of Group III-V compound semiconductor layers, for example, the crucible of one effusion cell would contain a Group III metal (e.g., liquid Ga), and the crucible of another cell would contain a Group V material (e.g., a solid source such as elemental As, or less commonly polycrystalline GaAs). On the other hand, in the CBE growth of such layers, one or more of the crucibles containing, for example, Group V material would be replaced by a gas source of, for example, arsine or phosphine. In either case, a third effusion cell or gas source would contain the source of a dopant. One consideration in the choice of a dopant is the conductivity-type of the layer to be grown. For example, to dope Group III-V compound layers n-type from a solid source tin (Sn) and silicon (Si) have been commonly used as dopants, and to dope such layers p-type from a solid source beryllium (Be) has been commonly used for many years. More recently, however, Be has been largely replaced by carbon (C).
Carbon has several characteristics that make it preferable as a p-type dopant in Group III-V compound layers deposited by MBE. First, Be is toxic; C is not. Second, Be has a relatively high vapor pressure and, therefore, during the high temperatures used in an MBE deposition processes, Be contaminates the growth chamber. Third, Be diffuses in the growing layer at a much higher rate than C. Therefore, precise control of the location and concentration of Be within very thin layers is difficult.
CBr4 is currently used in the industry to provide a source of C. See, for example, page 67 of the Product Guide 2000 of the EPI MBE Product Group, St. Paul, Minn., which is incorporated herein by reference. However, Br is corrosive, and extreme care must be exercised in evacuating it from the deposition chamber. Alternatives to CBr4 have been suggested in the prior art. For example, direct resistive heating of C filaments has been reported by R. J. Malik et al., J. Cryst. Growth, Vol. 127, pp. 686-689 (1993), which is incorporated herein by reference. Various methods for producing the C filaments have been tried including machining the filaments from a block of solid graphitic C or patterning them from a sheet of graphite foil. A. Mak et al., J Vac. Sci. Technol. B, Vol. 12, No. 3, pp. 1407-1409 (1994), which is also incorporated herein by reference, describe a woven filament that comprised a bundle of 6000, 10-μm-diameter graphite fibers. The fibers were clamped at both ends to a refractory metal support attached to an ultrahigh vacuum feed-through, as shown in FIG. 1 of the A. Mak et al. paper. The authors report a relatively short period of operation: only 15 hr at a power dissipation level corresponding to a hole concentration of 5×1018 cm−3 at 1 μm/hr growth rate. They also predict that repeated temperature cycling will shorten the filament lifetime.
We have found that, due to the relatively low resistivity of graphite filaments, they must be driven at relatively high input current levels to attain suitable doping levels. In addition, the high thermal conductivity of graphite filaments requires relatively high input power to attain requisite filament temperature. However, these high current and power levels tend to cause outgassing of the apparatus supporting the filament and of other components in the deposition system, which leads to undesirable contamination and, in turn, to decreased mobility of semiconductor layers grown in such systems.
On the other hand, a paper by R. J. Malik et al [Appl. Phys. Lett., Vol. 53, No. 26, pp. 2661-2663 (1988)] and the product literature of MBE Komponenten GmbH, Germany [MBE Komponenten, Dr. Karl Eberl, Products 2003, pp. 38-39] both describe a C sublimation source that utilizes a pyrolytic graphite, serpentine filament. Both of these references are incorporated herein by reference. However, pyrolytic graphite also has relatively high electrical and thermal conductivity, which means that correspondingly high power/current must be applied to generate suitable doping levels. In addition, the typical serpentine shape of the filament employed in these references suffers from hot spots at the sharp bends, which tends to decrease the filament lifetime.
As pointed out in the Komponenten literature, these issues of C doping also apply to the deposition of other than Group III-V compound layers; e.g., the deposition of Si—C and Si—Ge—C alloys.
Thus, a need remains in the MBE deposition art for a source of C doping that operates at lower power/current levels, and hence produces less contamination, and has a relatively longer lifetime than is currently available from graphite filaments.